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Photoinjector Activities at CERN

Photoinjector Activities at CERN. Christoph Hessler, Eric Chevallay, Steffen Doebert, Valentin Fedosseev, Irene Martini, Mikhail Martyanov (CERN) 20 February 2013 1 st Topical LA3NET Workshop on Laser Particle Sources, CERN. Outline. Introduction Photoinjector laser system

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Photoinjector Activities at CERN

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  1. Photoinjector Activities at CERN Christoph Hessler, Eric Chevallay, Steffen Doebert, Valentin Fedosseev, Irene Martini, Mikhail Martyanov (CERN) 20 February 20131st Topical LA3NET Workshop on Laser Particle Sources, CERN

  2. Outline • Introduction • Photoinjector laser system • Photocathode production • Beam measurements at the PHIN Photoinjector • Conclusion and outlook C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  3. CLIC C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  4. Photoinjectors at CTF3 Photoinjector laser lab (1st floor) and optical transfer line to PHIN and CALIFES Dedicated photoemission laboratory in Bldg 101 for photocathode production, testing and R&D C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  5. Motivation for a Drive-Beam Photoinjector • To generate the 12 GHz time structure, several fast 180 degree phase switches are needed, which is presently done by a sub-harmonic bunching system. • However, present system (thermionic gun, sub-harmonic buncher) generates parasitic satellite pulses, which produce beam losses. • Reduced system power efficiency • Radiation issues due to the beam losses of the satellite pulses • These problems can be avoided using a photoinjector, where the phase-coding can be done on the laser side and only the needed electron bunches are produced with the needed time structure. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  6. Laser-based Phase Coding Demonstration • Time structure: CTF3/CLIC drive beam requires several fast 180 degree phase-switches for beam combination (so-called phase coding). • Satellite-free beam production at PHIN using laser phase-coding based on fiber-modulator technology has been demonstrated in 2011. • Results: • Streak camera measurements of Cerenkov light:  switching time < 300ps  Satellites <0.1% M.Csatari Divall et al., “Fast phase switching within the bunch train of the PHIN photo-injector at CERN using fiber-optic modulators on the drive laser”, Nucl. Instr. And Meth. A 659 (2011) p. 1. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  7. PHIN and CLIC Parameters Main issues: - Long cathode lifetimes with high bunch and average charges - Laser system for CLIC parameters (UV generation, 50 Hz operation, stability) C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  8. Laser System To CALIFES photoinjector 4ω Harmonics 450μJ in 100ns (=3μJ / laser pulse) 2ω 3-pass amplifier Cooling 3.6kW 4.67mJ in 1.2μs 1.5 GHz Synched oscillator Cw pre-amplifier 10W Booster amplifier 320mW 3-pass amplifier 3.5kW 2-pass amplifier 8.3kW 2ω 4ω 7.8kW 14.8mJ in 1.2μs Harmonics test stand 1.25kW 1.5mJ in 1.2μs (=800nJ / laser pulse) HighQ front end AMP1 and AMP2 → More details in talk of Mikhail Martyanov To PHIN Photoinjector M. Petrarca et al., “Study of the Powerful Nd:YLF Laser Amplifiers for the CTF3 Photoinjectors”, IEEE J. Quant. Electr. 47 (2011), p. 306. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  9. Photocathode Production Laser beam Shutter • Co-evaporation of Cs and Te/Sb • Monitoring of each component by a separate microbalance (other component is shielded by a mask) • DC gun + diagnostic beam line for measuring the photocathode properties • Achieved QE: ~20% (Cs2Te), 7.5% (Cs3Sb) Te/Sb microbalance Photocathode plug Cs microbalance Evaporators Te/Sb evaporator Masks Cs dispenser E. Chevallay, “Experimental Results at the CERN Photoemission Laboratory with Co-deposition Photocathodes in the Frame of the CLIC Studies”, CTF3 Note 104, submitted C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  10. Co-deposition Cs3Sb Photocathodes • After stopping the evaporation, the QE first continues to increase during beam production. • Reason for this behavior still unclear, maybe due to re-organization of Cs and Sb atoms. • Maximum achieved QE: 7.5% Continuous beam operation C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  11. Photocathode History C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  12. PHIN Layout FCT: Fast current transformerVM: Vacuum mirror SM: Steering magnet BPM: Beam position monitorMSM: Multi-slit Mask OTR: Optical transition radiation screen MTV: Gated cameras SD: Segmented dump FC: Faraday cup C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  13. High Charge Production (Cs2Te) • Charge vs. laser energy scan with 50 ns long trains • Linear response up to 5 nC • Record bunch charge of 9.2 nC above CLIC requirements! • Close to the theoretical limit of Qmax=9.47 nC for a beam size of 1.8 mm s x 1.25 mm s 9.2 nC! Cathode #185 Cs2Te C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  14. Lifetime studies of Cs3Sb cathodes with green light • Measurements taken during PHIN run March 2012 • Excellent lifetimes obtained, much better than expected. • Long-time operation over 10 days with one cathode! • Operation of 1.2 µs long trains yield similar lifetime as for short trains. 2.3 nC, 350 ns,l=524 nm 2.3 nC, 1200 ns,l=524 nm 1/e lifetime 168 h 1/e lifetime 135 h C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  15. Lifetime Cs3Sb vs. Cs2Te Cathodes • Comparison with earlier lifetime measurements of Cs2Te cathodes. • Lifetimes are similar and within CLIC specifications. • For Cs3Sb a factor 6 less of QE is needed as for Cs2Te cathodes, due to the different wavelength and the absence of 4th harmonics conversion stage Cathode #189 (Cs3Sb) Cathode #185 (Cs2Te) 2.3 nC, 350 ns,l=524 nm 2.3 nC, 350 ns,l=262 nm 1/e lifetime 168 h (corresponds to 270 h above 0.5% QE) (corresponds to 300 h above 3% QE) C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  16. Impact of Vacuum on Cs3Sb Cathode Lifetime • Comparison with earlier measurements of Cs3Sb cathodes with UV light and worse vacuum conditions (same beam parameters). • Lifetime has drastically improved from 26 to 185 h. • Improved vacuum condition due to activation of NEG chamber around the gun. March 2012 March 2011 1/e lifetime 26 h 1/e lifetime 185 h 1 nC, 800 ns, l=262 nm 1 nC, 800 ns,l=524 nm 4e-9 mbar 7e-10 mbar C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  17. Impact of Vacuum on Cs2Te Cathode Lifetime Comparison of lifetime measurements with same beam properties but different vacuum conditions: • Substantial improvement of dynamic vacuum level has resulted in drastic increase of 1/e lifetime from 38 to 250 h. • Corresponds to total cathode lifetime of 300 h above 3% QE. l=262 nm l=262 nm Cathode #185 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  18. Conclusion • Cs3Sb photocathodes successfully operated at PHIN RF photoinjector with green laser light. • Similar lifetimes obtained as for Cs2Te cathodes.  Good candidate material for further studies • Good vacuum is mandatory for a good lifetime. • Cathode production process (co-evaporation) seems to be important for obtaining good lifetimes for Cs3Sb cathodes . C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  19. Outlook (1) Photocathode R&D: • Continue studies on new cathode materials sensitive to visible light at the photoemission lab and later at PHIN: • Cs3Sb • Multi-alkali photocathodes like e.g. K2CsSb, Na2KSb, … • Integrated charge studies with Cs3Sb cathodes and green light → New laser system installed • XPS surface analysis studies of photocathodes to get a better understanding of surface deterioration effects and the cathode life time → New transfer arm built in collaboration with LAL • Design and implementation of a load-lock system for fast cathode plug and evaporator exchange to increase availability of photoemission lab. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  20. Outlook (2) PHIN studies: • Continue studies of Cs3Sb cathodes using a green laser beam. • Push parameters as far as possible towards CLIC requirements (5 µs long pulse trains, 5 Hz repetition rate). • Further improvement of vacuum (Installation and activation of a new NEG pump). • Feedback tests with electron beam. Beyond PHIN: • However, the final proof of feasibility of a photoinjector for CLIC drive beam cannot be achieved with PHIN. • New 1 GHz RF gun specially designed for the CLIC requirements needed. • Looking for collaboration for designing this gun. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  21. Acknowledgement Collaborating institutes: … and thank you for your attention! LA3NET is funded by European Commission under Grant Agreement Number GA-ITN-2011-28919 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  22. Lifetime Studies of Cs2Te Cathodes • Cathode lifetime vs. vacuum • Correlation between lifetime and vacuum. • In high e-9 mbar/ low e-8 mbar < 50h lifetime was measured. • When vacuum is kept at low e-9 mbar lifetime is within specification. Cathode #182 C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  23. Improvement of PHIN Vacuum System Layout PHIN gun: Plan to improve vacuum in two steps: • Activation of existing NEG coated chamber around the gun(already done) • Activation of existing NEG coating in beam line and installation of additional NEG pump. Laser beam Electron beam Photo cathode 20 cm C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  24. XPS Surface Analysis Studies • Investigation of surface deterioration of photocathodes and its effect on quantum efficiency and lifetime • Planned to be performed in collaboration with CERN vacuum group • For the photocathode transfer to XPS lab a new transfer arm has been built in collaboration with LAL (Orsay) and will be ready soon: XPS apparatus Transfer arm C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  25. Studies for Load-Lock System in the Photoemission Lab • Implementation of existing horizontal and vertical manipulators for moving the cathode plug and the evaporator setup into a 3D CATIA model: • The next step is to implement a new vertical manipulator and a valve, which separates the evaporator setup from the preparation chamber. This will avoid the need of breaking the vacuum in the preparation chamber during an exchange of the evaporators. • Second step: Find a solution exchange cathode plugs without breaking the vacuum in the preparation chamber. S. Sroka S. Sroka C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  26. PHIN and CLIC Parameters C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  27. Phase Coding Demonstration • Motivation • CTF3/CLIC drive beam requires several fast 180 degree phase-switches for beam combination. • With currently used thermionic DC gun and sub-harmonic bunching satellites are produced, which could cause radiation problems. • Aim to provide an alternative satellite-free solution using laser phase-coding based on fiber-modulator technology and an RF photo-injector. • Setup: M.Csatari Divall et al., “Fast phase switching within the bunch train of the PHIN photo-injector at CERN using fiber-optic modulators on the drive laser”, Nucl. Instr. And Meth. A 659 (2011) p. 1. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  28. Phase Coding Demonstration • Results: • Streak camera measurements of Cerenkov light • Beam observation on fast current transformer:  switching time < 300ps  Satellites <0.1%  No beam degradation due to switching C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev, I. Martini, M. Martyanov

  29. Impact of Bunch Charge on Vacuum Vacuum vs. bunch charge • Vacuum can be maintained up to nominal bunch charge of PHIN of 2.3nC. • Pressure increase above nominal bunch charge probably due to losses inside gun. • A 1 GHz gun specially designed for CLIC might be able to maintain the vacuum up to a higher bunch charge due to larger apertures. C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev

  30. Impact of Train Length on Vacuum Corresponding beam losses Between FCT and Faraday cup Vacuum vs. train-length Bunch charge 2.3 nC • Vacuum correlated to beam losses in the beam line • When beam is optimized for good transport, the vacuum can be maintained with increasing trainlength C. Hessler, E. Chevallay, S. Doebert, V. Fedosseev

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